The present invention pertains to the bottom-blown oxygen steel-making method of converting impure molten iron to steel and is particularly concerned with safe and economic collection of the gases which are evolved in the process and which have to be cleaned before discharge to the atmosphere and/or used as fuel. The analysis of the gases is used to control the collection and gas cleaning.
In the bottom-blown oxygen steelmaking process, with which the present invention is concerned, the refractory lined converter vessel has tuyeres in its bottom so that oxygen, other gases such as hydrocarbon cooling fluids, and powdered fluxes and other additives needed for the conversion process may be introduced through the bottom of the vessel and diffused through the melt. This contrasts with the well-established top-blown oxygen conversion process where oxygen is injected into the melt by means of a lance which extends through the mouth of the vessel to near the surface of the melt. In the top-blown process, the fluxes and other materials are also admitted through the mouth of the vessel to the top surface of the melt.
In both the top and bottom-blown processes, oxygen (O.sub.2) reacts with silicon, manganese, carbon and phosphorus in the melt during the blowing period. In the bottom-blown process, oxygen (O.sub.2) also reacts with hydrogen resulting as a decomposition product from the hydrocarbon fluids injected in the bottom. the duration of blowing or end point of the heat is indicated by the reduction of carbon oxidation products (CO & CO.sub.2) in the evolved gases which means that the carbon content of the melt has been reduced to the desired level and that the melt may be poured provided that is temperature is correct.
In both processes gases are evolved during the blow period. The primary gases in the bottom-blown processes are carbon monoxide (CO), carbon dioxide (CO.sub.2), hydrogen (H.sub.2) and water vapor (H.sub.2 O) which may vary in proportion as the melt proceeds. In general these primary gases are directed into a hood over the vessel. The hood is spaced from the vessel so air enters and burns the CO and H.sub.2 in the gas and the products of combustion are conducted through an off-gas cleaning system with an exhaust fan, and then led to a chimney and discharged to the atmosphere as waste.
In such a combustion system, iron vaporized from the bath in localized high temperature zones, and such iron that is discharged as oxide, is completely oxidized to red fume (Fe.sub.2 O.sub.3) and diluted by combustion air and excess air. The gas cleaning system required to clean such fine red fume and high volumes of resulting off-gases is voluminous and costly due to high fan capacity and power consumption resulting from high pressure drop required to achieve air pollution standards.
The bottom-blown oxygen process in comparison to the top-blown, is characterized by a smaller percentage of iron vaporized but in an extremely fine dispersion of the iron particles. Therefore, a non-combustion system has to be used in order to avoid oxidation and further dilution with air, and to take advantage of the increased agglomeration and wetting properties of dust particles comprised largely of non or semi-oxidized iron (Fe, FeO and Fe.sub.3 O.sub.4) prevailing under reducing conditions. Besides the economic advantages of such a system, it provides the release of the lowest possible gas volumes with minimum concentration of dust in the exhausted stack gases. This ensures that all applicable pollution code standards can be economically met.
An associated objective of such a non-combustion gas cleaning system in conjunction with the bottom-blown oxygen steelmaking process, is the safe capture and possible utilization of relatively high calorific value off-gases produced during the blow. This is achieved through automatic switching in such manner that during the beginning and end of a blow, when off-gases have a relatively low calorific value, they are cleaned and discharged to a waste gas stack equipped with a torch burner; while during interim periods of the blow when the off-gases have a high calorific value they are cleaned and stored for external utilization. There are thus three distinct periods. A first period of exhaust and discharge to the atmosphere, a second period of collection of gases for use, and a third period when gases are again exhausted and discharged to the atmosphere. By controlling the duration of the second period of collection it is possible to obtain a gas the calorific value of which can be set at various desired values.
In this invention a scheme is proposed for avoiding combustion of the evolved gases in the bottom-blown process and particularly for collecting the gases when the CO and H.sub.2 concentration is about 30% at which time the gas mixture is combustible. This gas mixture is therefore subject to combustion and even explosion in the hood and gas cleaning system if sufficient oxygen is present. The problem with collecting the evolved reaction gases is that the gases are at about 1400.degree.-1600.degree. C at the mouth of the vessel and would further increase with combustion at the air gap between vessel mouth and hood. Gases must therefore be cooled sufficiently by radiation to about 1100.degree. C in order to reduce the steam content that would otherwise result from subsequent quenching in the wet gas cleaning system. Cooling is effected by using a water cooled hood and stack in order to lower the temperature of the gas mixture. If required, under special circumstances when unusually high CO and H.sub.2 evolution occurs early in the heat, nitrogen will be injected in the hood for inertization of the primary gases to a non-explosive level, and simultaneously providing the benefits of cooling.
The safe capture and efficient operation of the gas cleaning system and complete collection of gases therefore requires that combustion at the air gap between hood and vessel mouth be kept to a minimum after taking necessary precautions to ensure that no explosive conditions are prevalent in the system. Furthermore, controls are required to avoid leakage of gas and fumes to the atmosphere in the gap between vessel and hood resulting from process variations and fluctuations in evolved gas volumes within the vessel. Hitherto, this has been a major disadvantage in full combustion systems where such control is not possible and consequently adds to air pollution.